This invention relates to a molding thermoplastic, elastomeric resin composition having superior characteristics.
In recent years, thermoplastic elastomers having rubber elasticity even when uncured and also having thermoplasticity have been noted in the fields of parts for automobiles, domestic appliances and their parts, wire coating materials, footwears, materials for asphalt pavement, etc.
As such thermoplastic elastomers, polyolefin elastomers, polystyrene elastomers, etc. have so far been known.
Polystyrene elastomers, particularly styrene-butadiene block copolymer (SBS) and styrene-isoprene block copolymer (SIS) have a superior flexibility and a good rubber elasticity, but since they contain double bonds therein, their heat stability (heat deterioration and thermal-oxidative deterioration) and weather resistance are insufficient.
Thus, in order to improve such insufficient characteristics of the polystyrene elastomers, there have been employed or proposed .circle.1 a process of hydrogenating the double bonds inside the molecules of styrene-conjugated diene block copolymers, .circle.2 a process of reacting the double bonds within the molecules with each other by adding a free-radical generator or a crosslinking agent to form a reticular structure inside the molecules, and the like processes.
In any of these processes, it is intended to reduce the quantity of the double bonds inside the block copolymers or make it zero.
However, the thermoplastic elastomers obtained according to the process .circle.1 are superior in heat stability and molding and processing properties, but since the hydrogenation process relies on a solution process, the operation is complicated and the hydrogenation cannot be easily effected. Moreover, the hydrogenated thermoplastic elastomers are regarded as exhibiting not as good rubber elasticity as compared with unhydrogenated thermoplastic elastomers.
On the other hand, the thermoplastic elastomers obtained according to the process .circle.2 have drawbacks in that notable reduction in the flowability and inferior appearance of the resulting molded products, such as silver streaks, occur.
Further, as to the polyolefin thermoplastic elastomers, it has been known that when a hydrogenated derivative of styrene-butadiene-styrene block copolymers is blended with a propylene polymer, a composition having a good flexibility and also high mechanical strengths is obtained.
However, when a hydrocarbon oil used as a softening agent is further added to the above-mentioned composition in order to improve its flowability, the composition is reduced in mechanical strength.
Further, if propylene homopolymer is blended with the above-mentioned composition, good rubber-elasticity is hindered and high-impact properties at low temperatures are insufficient.
In the case of the composition of a propylene-ethylene block copolymer with a hydrogenated derivative of a styrene-conjugated diene copolymer, as the quantity of the hydrogenated derivative of a styrene-conjugated diene block copolymer blended increases, flow marks appear on the surface of the resulting molded product to yield a molded product having an inferior appearance.
On the other hand, in the case of a polyolefin elastomer obtained by blending a propylene polymer with a non-crystalline ethylene-propylene random copolymer, the elastomer has a good rubber elasticity, but its mold-release characteristics during the molding process are inferior, and in the case of those containing a large quantity of the non-crystalline ethylene-propylene random copolymer, there often occurs no mold release.